Edge-illuminated lightguides have been used in backlights for LCDs and more recently for light fixtures. For extended color, longer lifetime, increased optical efficiency, and cost, LEDs are becoming utilized more in backlight assemblies instead of CCFLs. Since LEDs are closer to being a point source, LED light can be controlled more efficiently than the extended source CCFL. However, by using the same white spot diffusers noted above in light guides, the light is scattered in all directions, up to the critical angle of the light guide air interface. The refracted angular spread of light out of the light guide can reach angles approaching 90 degrees from the surface. Additional diffuser films used to reduce the visibility of the white dots spread this light further into undesired, i.e., wider, angles. Optical films such as prismatic films are then necessary, to “rein in” a portion of this light back toward 0 degrees (the direction perpendicular to the surface). Thus, between the white dots spreading light out into larger angles than needed, and then using collimating films to bring a portion of this light back toward the normal or desired viewing angles, a significant amount of light is lost and the process is an inefficient one.
Other backlight configurations have been proposed using symmetric scattering particles instead of white dots. Scattering light guides have been described as “highly scattering optical transmission” (HSOT) polymers by Okumura et al (J. Opt. A: Pure Appl. Opt. 5 (2003) S269-S275). The authors demonstrated that a backlight based upon a HSOT polymer has the potential to provide twice the brightness of a conventional backlight. However, the particles used are symmetric or spherical in shape. The Okumura teachings do not account for the asymmetric nature of the input light, or the need for more light to be diffused vertically, horizontally, or out from the main face of the light guide. Also, traditional designs using planar lightguides such as used with LCDs have angular output, thermal, uniformity, efficiency, and form factor limitations. Light from light sources such as LEDs that are incident upon a volumetrically scattering lightguide can have significantly bright regions near the LEDs due to light directly reaching the volumetric scattering region and causing bright luminance non-uniformities on the light emitting surface near the edge of the lightguide. If the diffusion strength (angular FWHM intensity of the diffusion profile) of the volumetric light scattering lightguide is significantly reduced to try and prevent the high luminance non-uniformity near the edge, the optical efficiency of the lightguide is also reduced.